Study of superstructure II in multiferroic BiMnO₃

The crystal structure of the minor phase, named superstructure II, existing in multiferroic compound BiMnO₃ has been studied by electron diffraction and high-resolution transmission electron microscopy. Domains of major and minor phases coexisting in BiMnO₃ were observed in high-resolution electron microscope images. The unit cell of minor phase was determined to be triclinic with the size 4\times 4\times 4 times as large as the distorted perovskite subcell. The 111 and 10\bar1 projected structure maps of the minor phase have been derived from the corresponding images by means of the image processing. A possible rough three-dimensional (3D) structure model was proposed based on the 3D structural information extracted from the two projected structure maps. Since there is no inversion centre in the proposed model, the minor phase may contribute to the ferroelectric property of BiMnO₃.